1. Field of Invention
The present invention relates to a method and apparatus for dark level integration of an image sensor. More particularly, the present invention relates to a method and apparatus for dark level integration of an image sensor without integrating bad pixels.
2. Description of Related Art
For an image sensor, the signals read from the pixels of the image sensor have to be amplified. In general, as shown in FIG. 1, a programmable gain amplifier (PGA) 10 is used for amplifying the signal levels from the pixel. In order to further maximize the dynamic range of the possible ADC circuit, a dark level calibration integrator 20 is used for measuring and storing the dark current level. The dark current level takes place when the read out circuit of the image sensor is reading the pixel output from the dark rows.
In general, as shown in FIG. 2, the image sensor 30 contains dark rows 32, 34, 36 and 38. For example, the image sensor has a size of 352 row by 288 column. As shown in FIG. 3, which shows a timing diagram of the calibration signal CAL according to conventional method, when a dark row D appears, the calibration signal CAL asserts to enable the dark current calibration integrator 20. However, due to the current process, it is hard to fabricate an image sensor perfectly without any bad or defective pixel thereon. Therefore, there are always some defective pixels on the image sensor after it is fabricated. The dark row is also possible containing bad pixels therein.
According to the conventional method, if there are bad pixel in the dark rows of the image sensor, the dark level calibration integrator will average all of the pixel values including the bad pixels when the calibration signal is asserted. The calibration then causes significant errors
The invention provides an apparatus for dark level integration. The apparatus comprises a programmable gain amplifier, a calibration signal generator, and a dark current calibration integrator. The programmable gain amplifier (PGA) is used for receiving signals from pixels of an image sensor and for amplifying levels of the signals from the pixel. The calibration signal generator is used for receiving a first calibration signal and a bad pixel signal to generate a second calibration signal, wherein the second calibration signal indicates the existence of the bad pixel. The dark current calibration integrator couples to the programmable gain amplifier and the calibration signal generator for receiving the amplified signal levels from the programmable gain amplifier and the second calibration signal to generate a control signal to the programmable gain amplifier. The second calibration signal disables the dark current calibration integrator when the first calibration signal and the bad pixel signal have the same logic state.
The calibration signal generator mentioned above can consists of logic gates, for example an AND gate and an inverter. The inverter receives the bad pixel signal and outputs an inverted bad pixel signal to the AND gate, and the AND receives the first calibration signal and the inverted bad pixel signal to generate the second calibration signal.
The invention further provides a method for dark level integration. The method comprises the following steps: receiving signals from pixels of an image sensor; detecting a first calibration signal and a bad pixel signal; calibrating a dark current of the image sensor to generate a control signal when the first calibration signal and the bad pixel signal are not the same logic level; and amplifying the signals from the pixel in response to the control signal.
Accordingly, the present invention provides an easy and simple method and apparatus to avoid integrating the bad pixels into the dark current level of the image sensor. The invention can further reduce cost.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.